Dihydroxy-acid dehydratase (DHAD), also called acetohydroxy acid dehydratase, catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate and of 2,3-dihydroxymethylvalerate to α-ketomethylvalerate. DHAD-catalyzed conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate is a common step in the multiple isobutanol biosynthetic pathways that are disclosed, for example, in U.S. Pat. No. 7,851,188. Disclosed therein is engineering of recombinant microorganisms for production of isobutanol. Isobutanol is useful as a fuel additive, and the availability of biologically-produced isobutanol can reduce the demand for petrochemical fuels.